1. Field of the Invention
This invention relates to an austenitic stainless steel having excellent galling resistance in conventional wrought form, good wear resistance, good corrosion resistance in chloride-containing environments, excellent high temperature oxidation resistance, and a high work hardening rate. The alloy of this invention can be readily worked with conventional equipment into plate, sheet, strip, bar, rod and the like, and retains a substantially austenitic structure throughout a wide temperature range.
The steel of the invention is adapted to applications in which moving metal-to-metal contact and corrosive attack are encountered in combination. Although not so limited, the steel has particular utility for fabrication into roller chains, link belts on conveyors, valves subjected to elevated temperature, woven metal belts for continuous heat treating furnaces, fasteners, pins and bushings.
2. Description of the Prior Art
Although galling and wear may occur under similar conditions, the types of deterioration involved are not similar. Galling may be defined as the development of a condition on a rubbing surface of one or both contacting metal parts wherein excessive friction between minute high spots on the surfaces results in localized welding of the metals at these spots. With continued surface movement this results in the formation of even more weld junctions which eventually sever in one of the base metal surfaces. The result is a build-up of metal on one surface, usually at the end of a deep surface groove. Galling is thus associated primarily with moving metal-to-metal contact and results in sudden catastrophic failure by seizure of the metal parts.
On the other hand, wear is synonymous with abrasion and can result from metal-to-metal contact or metal to nonmetal contact, e.g. the abrasion of steel mining equipment by rocks and similar mineral deposits. Such wear is characterized by relatively uniform loss of metal from the surface, as contrasted to localized grooving with consequent metal build-up, as a result of rubbing a much harder metallic surface against a softer metallic surface. The distinction between galling and wear can perhaps best be illustrated by the fact that galling can be eliminated by mating or coupling a very hard metallic surface with a much softer metallic surface, whereas wear or abrasion in metal-to-metal contact would be increased by mating a very hard surface with a much softer one.
An article by Harry Tanczyn entitled "Stainless Steel Galling Characteristics Checked" in STEEL, Apr. 20, 1954 points out that stainless steel sections at a relatively high hardness level, or with substantial difference in hardness, exhibit better resistance to galling than the combination of two soft members. This may be explained by the theory that the hardened sections deform elastically near the contact points under loading, while the softer pieces yield plastically for a significant distance beneath the contact points. During movement, the hardened surfaces apparently recover elastically with decrease in pressure, and this motion tends to sever any metallic welding. This article also indicates that good resistance to galling may be traceable to the combination of a suitable oxide surface film and a hard backing. Oxide films were found to influence the galling characteristics of metals, e.g. a film of Fe.sub.3 O.sub.4 increased the resistance of mild steel to galling, while a film of Fe.sub.2 O.sub.3 did not benefit resistance to galling.
Among the numerous prior art steels currently available, the austenitic AISI Type 304 is suited to a variety of uses involving welding and fabrication, but the galling and wear resistance of this steel are poor, and the metal is likely to fail when subjected to such conditions.
A precipitation-hardening stainless steel, sold under the registered trademark ARMCO 17-4 PH (about 16.5% chromium, about 4.0% nickel, about 4.0% copper, about 1.0% manganese, about 1.0% silicon, up to 0.07% carbon, 0.35% columbium, and remainder iron), while possessing high strength and hardness in the hardened condition, exhibits only fair galling and wear resistance.
U.S. Pat. No. 3,663,215, issued May 16, 1972 to H. Tanczyn, discloses a steel having improved wear resistance, which at the same time is weldable, workable, and/or machinable, and precipitation hardenable by heat treatment to great hardness. It has been found that this steel has good galling resistance. However, it contains large amounts of expensive alloying elements, and it is difficult to process with standard steel mill equipment. The broad composition ranges are about 10% to about 22% chromium, about 14% to about 25% nickel, about 5% to about 12% silicon, one or more of the elements molybdenum up to about 10%, tungsten up to about 8%, vanadium up to about 5%, columbium up to about 5% and titanium up to about 5%, these additional elements being in sum total of about 3% to about 12%. Carbon is present up to about 0.15% and nitrogen up to about 0.05%. In this alloy silicon is stated to form silicides of molybdenum, tungsten and the like, in finely dispersed form in the matrix of the precipitation-hardened steel. These silicides are of extreme hardness, thereby providing good wear resistance.
A prior art steel presently considered to have the best resistance to wear and galling is the straight chromium AISI Type 440C, containing about 16% to 18% chromium, about 1% maximum manganese, about 1% maximum silicon, about 0.75% maximum molybdenum, about 0.95% to 1.20% carbon, and remainder iron. This steel is hardenable by heat treatment but has poor corrosion resistance and poor formability. It is difficult to roll into plate, strip, sheet, bar or rod, and articles of ultimate use cannot be readily fabricated from plate, sheet, strip, bar or rod form.
Reference is further made to Ser. No. 445,482 filed Feb. 25, 1974 as a continuation of Serial No. 238,862 filed Mar. 28, 1972, now abandoned (as a C-I-P of Ser. No. 868,893 filed Oct. 23, 1969, now abandoned) in the names of George N. Goller and Ronald H. Espy and assigned to the assignee of the present application. This application discloses and claims an austenitic stainless steel having excellent stress corrosion cracking resistance, good weldability, good cryogenic strength and toughness, and high strength at room temperature resulting from a high work hardening rate. This alloy has been found to possess good galling and wear resistance. Its broad composition comprises from about 15.5 to about 20% chromium, from about 11% to about 14% manganese, from about 1.1% to about 3.75% nickel, from about 0.01% to about 0.12% carbon, from about 0.20% to about 0.38% nitrogen, up to about 1% silicon, up to about 0.06% phosphorus, up to about 0.04% sulfur, and remainder substantially iron.
From the above background of the present state of the art, it is apparent that there is not now available an alloy having excellent galling resistance in wrought form, good wear resistance, good corrosion resistance to chloride-containing environments, good high temperature oxidation resistance, and which is readily workable into plate, sheet, strip, bar, rod and like wrought products.